FIG. 2 is a sectional front view of a prior art solid electrolytic capacitor 1, and FIG. 1 is a perspective view of a prior art capacitor element 2 (see, for example, Japanese Examined Patent Publication No. HEI4-19695 (1992)).
The solid electrolytic capacitor 1 includes an aluminum case 3 having a top opening, the capacitor element 2 contained in the case 3, and a rubber packing 30 which seals the opening of the case 3. An upper edge portion of the case 3 is curved to fix the packing 30, and a plastic seat plate 31 is attached to the top of the case 3. Lead wires 21, 21 extend from the capacitor element 2 through the packing 30 and the seat plate 31, and then bent laterally.
As shown in FIG. 1, the capacitor element 2 includes an anode foil 4 of an aluminum foil coated with a dielectric oxide film and a cathode foil 5 of an aluminum foil, which are rolled together into a roll with a separator 6 of an insulative material such as paper interposed therebetween. The capacitor element 2 includes an electrically conductive polymer layer provided therein. Lead tabs 25, 25 respectively extend from the anode foil 4 and the cathode foil 5, and the lead wires 21, 21 respectively extend from the lead tabs 25, 25.
A process for forming the electrically conductive polymer layer in the capacitor element 2 will be described below. First, a polymer material of thiophene is dissolved in an alcohol solvent such as ethyl alcohol, and an oxidizing agent such as a metal salt is added to the resulting solution. Then, the capacitor element 2 is immersed in the solution, followed by thermal polymerization at a temperature ranging from a room temperature to about 300° C. Thus, the electrically conductive polymer layer is formed in the capacitor element 2.
Cations in the oxidizing agent are incorporated as a dopant in the polymeric structure of the polymer layer to generate positive holes in the polymer layer, so that the polymer is imparted with the electrical conductivity. Although the solid electrolytic capacitor employing polythiophene as the electrically conductive polymer is well known (see, for example, Japanese Unexamined Patent Publication No. HEI2-15611 (1990)), pyrrole or aniline may be used as the polymer material. It is also known that polyethylenedioxythiophene is employed as an electrolyte and iron(III) p-toluenesulfonate is employed as the oxidizing agent (see, for example, Japanese Unexamined Patent Publication No. HEI9-293639 (1997)). The polymerization speed for polyethylenedioxythiophene is moderate, so that an electrolyte layer can be uniformly formed from the electrically conductive polymer in the capacitor element 2.
There is a market demand for reduction of ESR (equivalent series resistance) of the capacitor of this type. The capacitor which employs polyethylenedioxythiophene as the electrolyte is conventionally used, but does not have ESR characteristics satisfying the market demand. Further, the capacitor 1 of this type has great variations in capacitance and life test results. This is supposedly because the electrolyte layer is not sufficiently densely and uniformly dispersed in the capacitor element 2.